Stepless compensation of reactive current



Dec. 26, 1961 M. SANGL ETAL 3,015,059

STEPLESS COMPENSATION OF REACTIVE CURRENT Filed June 15, 1959 5Sheets-Sheet 1 Dec. 26, 1961 M SANGL ETAL 3,015,059

STEPLESS COMPENSATION OF REACTIVE CURRENT Filed June 15, 1959 3Sheets-Sheet 2 Dec. 26, 1961 M. SANGL ETAL STEPLESS COMPENSATION OFREACTIVE CURRENT Filed June 15, 1959 3 Sheets-Sheet 3 Patented Dec. 26,1961 3,015,059 STEPLESS COMPENSATION OF REACTIVE CURRENT Michael Sang]and Werner Elischer, Erlangen, Germany, assignors to P. Gossen & Co.G.m.b.I-I., Erlangen, Bavaria, Germany Filed June 15, 1959, Ser. No.820,527

Claims priority, application Germany Sept. 6, 1958 2 Claims. (Cl.323-101) The invention pertains to a device for the stepless and,particularly, automatic compensation of a continuously changinginductive or capacitive reactive current by a controllable reactiveload.

Inductive or capacitive reactive currents, varying comparatively rapidlyand continuously in magnitude, occur in various electric installationsowing to the specific characteristics of the operation involved. This isthe case, for example, in an induction melting furnace, where thematerial to be melted is inside a winding carrying alterhating current.The inductive reactance of this winding is subject to considerablefluctuation in the melting of ferromagnetic, paramagnetic, anddiamagnctic substances because of the changes of the ohmic resistanceand the magnetic conditions within the material being melted. To secureas high a possible power consumption-by the melting furnace or torelieve the load on the input leads and the generator, the continuouslyvarying inductive current of the furnace must be continuouslycompensated by capacitances in parallel. Up to the present time theeffective parallel capacitance has had to be adapted to the condition ofthe melt by connecting or disconnecting capacitances by means ofcontactors, by hand, or by automatic switchgear.

It is obvious that such switchgear is of complicated design and subjectto trouble as the result of wear of the moving parts. This requirescontinuous or at least regular maintenance of such installations.Furthermore, the contactors and capacitances must meet particularly highrequirements, as switching is done with full voltage across the switchcontacts. This entails a considerable expenditure for the variouscapacitances and contactors. Moreover, the induction furnace reactivecurrent can be compensated only in steps, that is not exactly enough.

The reactive current compensation of the invention, on the other hand,consists entirely of stationary and sturdy parts that are not subject toany wear. The expensive contactors that are othervn'se required areeliminated completely, and the entire capacitance, which heretofore hasbeen subdivided into several units, may be combined into a fixed unit.Furthermore the arrangement of the invention makes it possible tocompensate the reactive current continuously and in such a way thatthere is a small capacitive component in the line.

This is achieved by connecting a fixed capacitance in parallel with theinduction melting furnace, which is able to compensate the maximuminductive reactive current of the furnace, and, secondly, by usingaspecial regulating reactor as a sort of current-regulating magneticamplifier. This regulating reactor, controllable by direct current,automatically and continuously compensates the resultant capacitivereactive current in combination with a circuit for measuring thereactive current I -sin Other advantages and objects of the inventionwill be made apparentby the following description and the accompanyingdrawing; wherein, as an example, the invention is applied to aninduction melting furnace, as mentioned above. 7

FIG. 1 is a basic circuit diagram of the invention.

FIG. 2 illustrates the construction of the transductor.

FIGS. 3a, 3b and 4 are diagrams explanatory of the operation of theinvention.

FIG. 5 is a diagram of an automatic compensating ar rangement accordingto the invention.

According to the basic circuit diagram of FIG. 1 the controllableinductance is embodied in the load winding 1, connected in parallel withthe furnace winding 2. In parallel with the latter there is thecapacitance 3. These circuit elements are connected to the A.C. source5-, a medium-frequency generator for, say, 10 kilocycles, via the lines4 and 4 The capacitance 3 is dimensioned so as to compensate the maximuminductive current of the furnace.

The mechanical construction of the transductor consisting of the twoparts T and T is shown in FIG. 2. It consists of the two adjacent ironcores 6 and 7, which are designed as continuous-strip toroidal cores,the load winding 8 and 9, and the control winding 10, which is woundaround the two other legs of cores 6 and 7 for both. The load windingconsists of two parts, wound in opposite directions, and connected inparallel. As the same A.C. voltage is impressed on both parts of theload winding, the actions of the induction fluxes flowing in the centerlegs of the core upon the control winding 10 cancel out.

To explain the mode of operation two idealized magnetization curves forthe two transductors T and T respectively, are shown in FIG. 3a. TheD.-C. biasing should be chosen so that when there is no A.C. voltageapplied, core 6 is saturated negatively at point P and core 7 saturatedpositively at point P When A.C. voltage is impressed across the loadwinding, an additional alternating flux is superposed on the constantbiasing D.-C. fiux in each transductor.

The cores of the transductors and the load windings 8 and 9 are sodimensioned with respect to the A.C. voltage, that when no biasmagnetization is present, they are not saturated by the A.C. voltage.

When polarization (biasing) is present, on the other hand, themagnetization curves are no longer traversed symmetrically with respectto the H-axis, but the A.C. fluxes are shifted in the direction P or Pdepending upon the nature of the biasing, so that we get the sections ofthe oscillation curves of the two transductors (magnetic amplifiers),started at the time t shown in FIG. 3b, 11 denoting the A.C. voltageapplied, and i and i denoting the currents in windings 8 and 9,respectively.

During the positive half-wave of voltage the current i can only rise tothe value of the control current in the load winding of T As the voltagecontinues to rise, the core of T emerges from the region of saturation,because the saturating action of the control voltage is exact- 1ybalanced out in this case. Because of the very high inductance of theload winding along the steep portion of the demagnetizationcharacteristic, the current i remains constant to begin with, until thefield has again been displaced so far into the negative region towardthe end of the subsequent negative half-wave that the lower break in thecurve is reached. When this is so the coreof T is again saturated, and ijumps to a high value, which depends upon the ohmic resistance of theload winding 8, on the one hand, and upon the degree of polarization, onthe other, for a given voltage a. If this polarization (biasing) isgreater than that shown in FIG. 3a, the lower break in the magnetizationcharacteristic isreached sooner as the field is reversed, and the timeduring which the core of T is saturated becomes longer, so that we get alarger current-time area. In principle, the same conditions prevail inthe right-hand curve shown in FIG. 3b. Because of the direction of theturns in winding 9, which is opposite t'o-that of winding 8, the regionis reached during the positive voltage half-wave, so that at the end ofthe latter current pulses flow in the positive direction. The constantcomponents of the currents i and i which depend upon the degree ofbiasing and have already been mentioned, flow in opposite directions, sothat their effects cancel out in the external circuit.

As the current pulses always flow at the end of the correspondingvoltage half-wave, this is equivalent to an inductive phase displacementof nearly 90..

The currents and voltages present in the furnace connected together withthe capacitance and the regulating reactor are shown in FIG. 4 for thecase where the amplitude of the capacitive reactive current i is some50% greater than that of the inductive reactive current i Accordingly, aresultant capacitive reactive current will flow in the lines to thegenerator. As is readily seen in the figure, the current pulse icompproduced by the compensation circuit described coincides in time withthe opposite half-wave of the capacitive current. Thus, compensationtakes place. To secure complete compensation,

the polarization (biasing) is so chosen that the currenta time area ofthe compensation pulse, together with that of the half-wave of theinductive reactive current, equals the current-time area of thecapacitive half-wave. Hence, the regulating reactor described makes itpossible to achieve stepless compensation of the capacitive reactivecurrent by means of a simple and inexpensive D.-C. control.

There is no external resistance connected in series with the loadwinding of the regulating reactor. Hence the amplitude of the currentpulses through it depends only upon its own ohmic resistance forconstant polarization. Moreover, since the ends of these impulses alwayscoincide more or less'with the passage of the voltage curve through zerofor different values of polarization, the shorter and steeper the pulsesare for equal current-time areas, the better the 90phase displacement.Hence the ohmic resistance of the load winding is kept as low aspossible.

In FIG. 2 the two legs of cores 6 and 7, which have no windings 8 and 9on them are wound with a common control winding 10. As a result, asalready mentioned briefly in connection with this figure, the effects ofthe A.-C. fluxesinthe adjacent cores are cancelled out with respect tothe control circuit. To be sure, this would also be the case in theusual division into two spatially separated coreswith their own controlwindings. But the disadvantage of such an arrangement is that a highinduced. is produced across each control winding, which is cancelled outexternally only as the result of the connection of the two controlwindings in opposite directions. The resultant high potential acrosstheinsulation of the two windings is avoided in the setup of FIG. 2,because here the A.-C. fluxes flowing in the cores are in oppositedirections. This already provides a sort of magnetic compensation.

The connections of'the reactive current load of the invention with anl-sin meter and with a magnetic amplifier connected inuseries with thelatter to constitute an automatic reactive current compensator is shownin FIG. 5.

The circuit for measuring 1 sin qt consists of the current transformer11, which is connected on its secondary side with another currenttransformer 12, the rectifiers 13, 14, 15, and 16, the condenser 17, andthe two condensers 1S and 19 connected in series. A rectifier 20 as wellas the control windingsof 'a magnetic amplifier 21 are connected to theoutput end of this'I-sin g measuring circuit, formed by theterminalsofthe'series'circuits 18 and 19. Another rectifier 22 isconnected in the line to these control windings; The rectifiers 13-16constitute a remote-controlled switch, being opened and closed by thevoltage across the lines 4 and 4;. Either rectifiers 13 and 14 areopened and rectifiers and 16 closed, or vice versa, .iepending'upontheinstantaueous polarities. of the control voltage. Accordingly, thecontrol voltage opens up two different current paths for the secondarycurrent in transformer 12. But as any change in these current paths alsochanges the polarity of the secondary current in the current transformer12, a pulsating DC. current flows through the elements connected to theoutput ends of the I -sin measuring circuit.

The phase of the control current produced by the voltage across thelines 4 and 4 is displaced through by the capacitor 17 and by thecapacitors 18 and 19 that act in parallel in this case. Hence thismeasuring circuit delivers maximum current to the subsequent elementswhen the reactive current in lines 4 and 4 is at a maximum, i.e. I -sin=I. The polarity of the pulsating D.-C. current at the output ends ofthe I-sin measuring circuit just described depends upon whether thereactive current flowing through the lines 4 and 4 is capacitive orinductive. Rectifiers 20 and 22 act so as to allow a control current toflow through the control windings of the transductor (magneticamplifier) 21 only when this reactive current is capacitive.

The control winding 10 of the regulating reactor of the invention, whoseoutput windings 8 and 9 are connected in parallel with the furnacewinding 2, serves as a load for the'magnetic amplifier 21, beingconnected to the latter through the rectifier bridge 23, which containsa smoothing capacitor, not shown.

The mode of operation of this circuit is readily understood frornwhathas been said above. If the inductive reactive current in thefurnace'windings drops during the melting process, a resultantcapacitive reactive current flows in lines 4 and 4 This generates aD.-C. voltage at the output of the I -sin measuring circuit, whichcauses current to flow through the control winding of the magneticamplifier 21. The control parameter is amplified in the latter, thencontrolling the current flowing through the load windings 8 and 9 of theregulating reactor in such a way as to nearly completely compensate thecapacitive reactive current, establishing a state of equi librium with asmall and desired capacitive residual current. The large capacitance inparallel with the furnace winding effectively short-circuits the upperharmonics generated by the pulse-shaped currents through the regulatingreactor.

If the input amplifier 21 is provided with an additional controlwinding, and a D.-C. current that is fixed or is proportional to. theapparent current flows through the line, the reactive currentcompensator regulates the line current to a fixed reactive current I sin4) that corresponds to the given direct current in one case, while inthe other it regulates the line current to any desired power factor cos4:. This supplementary winding is required whenever a power factor iswanted on the line that differs from unity.

Small reactive currents can also be controlled by employing a so-calledinductance-controlled regulating reactor, in which the working point isshifted back and forth by the biasing control parameter within thesteeper and not yet fully saturated regions of the magnetizationcharacteristic curve. This arrangement has the advantage of beingparticularly free of upper harmonics.

For high frequencies a core material formed of ferrite is advisableinstead of the cores 6 and 7, which are preferably made of thin ironstrip in the present case.

What is claimed is:

1. Apparatus for continuously compensating for reactive current in analternating current line connected to an inductive load, comprisingcircuit means for producing a direct currentwhich is a measure of. thereactive component of the line current, said means including a rectifierswitching circuit, an input transformer connected to the input of' saidrectifier circuit, a current transformer connecting said alternatingcurrent line to the primary winding of the input transformer, acapacitorconnecting one side of said line to a midpoint of the secondary of theinput transformer, and means for connecting the midpoint of the outputof said rectifier circuit to the other side of the alternating currentline, capacitor means connected in parallel with said load, saidcapacitor means having a greater magnitude than that required tocompensate for the inductive load, a magnetic amplifier, said magneticamplifier including two contiguous ring cores, a control winding Woundaround both cores, and a load winding connected across said line andsaid control winding, said load winding comprising two coils connectedin parallel and wound on each of said cores, respectively, to produceop- 10 posing magnetic fields in said control Winding, and means forimpressing said direct current on said control Winding with a polaritysuch that the magnetic amplifier is controlled only in response tocapacitive reactive current on said line.

2. Apparatus according to claim 1 including a second magnetic amplifierhaving load windings and a control Winding, a rectifier connecting saidlast-mentioned control Winding to the output of said rectifier circuit,and a second rectifier connected in series with the load windings of thesecond magnetic amplifier across the line, the output of said secondrectifier being connected to the control winding of the first-mentionedmagnetic amplifier.

References Cited in the file of this patent UNITED STATES PATENTS1,227,302 Osnos May 22, 1917 1,836,886 Thompson Dec. '15, 1931 2,421,786Haug June 10, 1947 Rhyne May 21, 1957

